The invention relates to a device for the inspection of surfaces as disclosed in the introductory part of claim 1, and to a method as disclosed in the introductory part of claim 13.
U.S. Pat. No. 5,923,423 teaches the use of a device for the inspection of surfaces which includes a laser light source whose emitted beam is first split into two sub-beams in a beam splitter, one sub-beam being applied to a photodetector as a reference beam whereas the other beam is applied to the surface to be inspected as a measuring beam. The measuring beam has a direction which essentially opposes that of the surface movement. The light reflected by the surface is superposed on the reference beam in the further optical beam path (heterodyne detection). A space resolution detector (an array of, for example, photodiodes) provides the localization of surface defects on the surface. This requires very accurate adjustment of the detector relative to the surface. The resolution is limited by the detector, for example a CCD camera.
It is an object of the invention to enhance the measurement of contaminations or damage on surfaces.
The object is achieved in accordance with the invention by means of a device that includes an evaluation unit for determining the velocity (v) of a defect on the surface from the shifted frequency (vxe2x80x2) and from this velocity the position of the defect on the surface, by means of a method where the speed of a defect on the surface is determined from the superposed signal formed from the at least one reference beam and the reflected light, and that the position of the defect on the surface is determined therefrom, and by means of an evaluation unit with a computer program that determines the frequency of the input signal from an alternating voltage component thereof, compares this frequency with a reference and calculates therefrom, by way of the Doppler formula, the velocity that corresponds to the frequency difference between said signals.
Because of the construction of the device in accordance with the invention, in which a detected defect is localized by way of a Doppler frequency shift, a speed that is determined therefrom, and an arithmetical determination of the position that is thus enabled, the location can be very accurately determined. This results in a very high resolution of the device that also enables the detection of small defects. Moreover, the speed of measurement is very high.
Defects occurring on the surface (for example, a particle resting thereon or a damage area that extends inwards (pinhole)), are irradiated in motion by the component of the light beam that extends in the direction thereof or in the opposite direction, so that the frequency of the light that is reflected by the defect is changed relative to that of the applied light. The use of the Doppler effect significantly simplifies the measurement of the location. The detector need not satisfy very severe requirements. Moreover, the laser need not oscillate in a defined mode either. The frequency shift that is induced between the applied light and the reflected light by the speed of the defect lies in the range that can be readily measured, that is, typically in a range of the order of magnitude of a few 10 kHz, thus facilitating the evaluation.
The resolution is significantly finer than the size of the light spot. Therefore, despite the desired high resolution a comparatively large light spot can be used so that the time required for scanning the surface is reduced.
When an elliptical light spot is formed with a major axis that extends radially in the case of a round or practically round wafer to be inspected, defects in different radial positions on the surface to be inspected will traverse different regions of the light spot. Because the defects that are situated further outwards in the radial direction have to travel a distance per unit of time that is larger than that traveled by defects that are situated further inwards, their speed is increased; this increase can be measured by way of the Doppler shift of the frequency. The radial position of the defect can thus be localized. The invention thus enables exact determination of the position of a detected defect by way of a velocity measurement that is carried out on the basis of the Doppler frequency shift.
There is also envisaged detection of the instantaneous orientation of the surface, for example, in a decoder associated with the rotary drive of the wafer, so that additionally, for example, the angular position of a wafer can be determined upon detection of a defect. The pole co-ordinates of the defect are thus fully known. Exact localization is then possible.